An animal model system has been established with C57BL/6J mice to study the influence of protein-calorie malnutrition on the primary immune response during the early postnatal development of the immune system. The present study will examine the primary antibody response of protein-calorie malnourished C57/BL/6J mice and impairments at the cellular level will be defined by methodology designed (1) to quantitate the primary immune response, (2) to measure the proliferative capacity of immunocompetent cells, (3) to measure the frequency of immunocompetent cells, and (4) to define defects inherent to cell populations or in physiologic environment.